Such apparatuses, also referred to as vibronic sensors, are applied as fill level measuring devices and have frequently an oscillatory fork as an oscillatable unit. However, also variants with a single rod or a membrane are known. The oscillatable unit is excited during operation by means of an electromechanical transducer unit to execute mechanical oscillations. The electromechanical transducer unit can be, for example, a piezoelectric drive or an electromagnetic drive.
Of course, besides the mentioned examples, also other options are available, which likewise fall within the scope of the present invention.
Corresponding field devices are sold by the applicant in many different forms. In the case of fill-level measuring devices, for example, devices of such type are sold under the marks, LIQUIPHANT and/or SOLIPHANT. The underpinning measuring principles are known from a large number of publications. The exciting of the oscillatable unit can be performed both by means of analog as well as also digital methods and occurs most often via an analog, electrical, oscillatory circuit. The electromechanical transducer unit excites the oscillatable unit by means of an electrical, exciter signal to execute mechanical oscillations and receives the oscillations and transduces them into an electrical, received signal. The electromechanical transducer unit can be either separate driving- and receiving units or a combined driving/receiving unit.
In such case, the electromechanical transducer unit is part of a control loop integrated in an electronics unit. The control loop sets the exciter signal in the normal case in such a manner that a predeterminable phase shift is present between the exciter signal and received signal. For example, for resonant oscillation, the oscillatory circuit condition must be fulfilled, according to which all phases arising in the oscillatory circuit yield a multiple of 360°.
Both exciter signal as well as also received signal are characterized by frequency, amplitude and/or phase. Therefore, changes in these variables are usually taken into consideration for determining the respective process variable, such as, for example, a predetermined fill level of a medium in a container, or also the density and/or viscosity of a medium. In the case of a vibronic limit level switch for liquids, it is distinguished, for example, whether the oscillatable unit is covered by the liquid or is freely oscillating. These two states, the free state and the covered state, are distinguished, in such case, based on different resonance frequencies, thus a frequency shift. Density and/or viscosity, in turn, can be ascertained with such a measuring device only in the case of an at least partial covering with the medium.
Used as exciter signal are usually sine- or rectangular signals. The advantage of a sinusoidal signal is that no, or few, overtones, which are whole-numbered multiples of the exciting signal, are transmitted to the oscillatable unit. In this way, the provided oscillatory energy is advantageously used essentially for only one oscillatory mode. With reference to the signal production within the electronics unit, this is, however, comparatively complicated and associated with a comparatively higher power consumption of the measuring device. Therefore, expediently, in general, a rectangular signal is used for the excitation. This enables, especially, that vibronic measuring devices can be operated via 4-20 mA- or NAMUR-interfaces.
A disadvantage of using rectangular signals, however, is that, besides the excitation frequency, also overtones are transmitted to the oscillatable unit. As a result, considerable noise can emanate from the oscillatable unit.
For the majority of all applications, the desired oscillatory mode is the fundamental oscillation mode, which is excited by exciting with the fundamental resonance frequency. However, also known are applications, in the case of which the torsion mode is excited.